Various types of rechargeable battery packs are known for use with hand-held electronic equipment such as video camcorders, notebook computers and especially cellular telephones. The vast majority of such battery packs use either Lithium cells, or nickel-cadmium or nickel-metal hydride cells. The battery packs may be provided with voltage control and protective circuitry for ensuring correct charging and discharging conditions to permit proper operation of the battery in conjunction with the electronic equipment.
Battery packs containing nickel-based cells generally contain the number of cells required to provide the operating voltage for the equipment, and there is therefore no necessity for voltage conversion circuits, but only for over-temperature and short-circuit current protection circuits. In battery packs containing Lithium cells, voltage conversion and control circuitry are not usually incorporated. However, Lithium cell battery packs must include voltage and current protection devices in addition to the standard components contained in Nickel-based cells.
Battery packs were often provided in the past as a single unit incorporating the batteries together with its associated circuitry. A more recent trend has been to install these circuits within the equipment, and to limit the electronic circuitry contained in the battery pack itself to basic protection for over-temperature and short-circuit only.
There is a marked lack of standardization with battery packs. Even in a single product area, such as that of cellular telephones, each battery pack is generally designed for a specific manufacturer and for specific models made by each manufacturer. As a result, both because of the market-driven factors of manufacturer/model specificity of the various battery packs and the more limited production quantities for each type, and because of the additional production expense of incorporating any electronic circuitry, the cost of such battery packs is high.
Furthermore, since the majority of such electronic equipment is designed to operate at voltages which are high, such as 3.6 volts, 4.8 volts and 6 volts, in comparison with the 1.2 volt single cell voltage of the generally used nickel cadmium (NiCd) or nickel metal hydride (NiMH) cells, most conventional battery packs utilize 5 cells to provide the necessary working voltage, as mentioned above. They are therefore large, both in size and in weight.
As a result of the high cost and the low convenience of the above mentioned prior art battery packs, many users do not carry spare battery packs with hand-held electronic devices. This is especially true for cellular telephones, as they are often carried constantly on the user's person.
There are known in the prior art several examples of battery charging circuits designed for integral use with rechargeable battery packs. In U.S. Pat. No. 5,218,284, a switching regulator circuit optimizes the charging rate and regulates current flow to the load.
In GB Patent No. 2,270,793, a DC--DC converter is used for stabilizing the voltage supply of a Lithium cell battery pack as the cells become progressively more discharged in use.
GB Patent No. 2,285,188 describes a power management system for a battery recharging circuit, wherein a charge regulator circuit controls the rate of charge of the battery.
A battery charger having an electronic switch for controlling charge and discharge is disclosed in PCT patent application WO 9515017, and enables conditioning of the battery, improvement in performance and lifetime, and monitoring of the battery temperature.
The battery pack control circuits designed for cellular telephone use are different from those used for other portable electronic equipment, in that they generally include a stand-by circuit, operational when the mobile phone is on but not being used. The most recent circuits of this type are "burst control circuits", so called because they are quiescent for most of the time, but turn themselves on for a very short "burst" of time at regular intervals, in order to check whether a transmission is being received. They typically turn on to check for transmission every few milliseconds, and with a typical duty ratio of 5%. The average current consumption of such a burst control circuit is therefore very low. The battery life whilst the phone is not in use is largely dependent on the effectiveness of this circuit.
Burst control circuits are currently implemented using dedicated commercially available IC's. However, such prior art burst control circuits have a disadvantage in that they are not bi-directional. Consequently, they cannot be used with voltage control circuits, such as those described in the prior art, which are used for controlling both the charging and discharging cycles of a battery pack. This is a major disadvantage for use in rechargeable battery packs for cellular phones.
None of the prior art battery pack control circuits are thus able to provide two way DC voltage multiplication, both during charging and discharging cycles, so that a battery with a different number of cells than required for operating the portable device will be seen both to the charger and to the load as if it had the correct number of cells, and at the same time, provide a long stand-by battery lifetime by virtue of including an effective two way burst control circuit.